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  <div class="section" id="module-queue">
<span id="queue-a-synchronized-queue-class"></span><h1>17.7. <a class="reference internal" href="#module-queue" title="queue: A synchronized queue class."><code class="xref py py-mod docutils literal"><span class="pre">queue</span></code></a> — A synchronized queue class<a class="headerlink" href="#module-queue" title="Permalink to this headline">¶</a></h1>
<p><strong>Source code:</strong> <a class="reference external" href="https://github.com/python/cpython/tree/3.6/Lib/queue.py">Lib/queue.py</a></p>
<hr class="docutils" />
<p>The <a class="reference internal" href="#module-queue" title="queue: A synchronized queue class."><code class="xref py py-mod docutils literal"><span class="pre">queue</span></code></a> module implements multi-producer, multi-consumer queues.
It is especially useful in threaded programming when information must be
exchanged safely between multiple threads.  The <a class="reference internal" href="#queue.Queue" title="queue.Queue"><code class="xref py py-class docutils literal"><span class="pre">Queue</span></code></a> class in this
module implements all the required locking semantics.  It depends on the
availability of thread support in Python; see the <a class="reference internal" href="threading.html#module-threading" title="threading: Thread-based parallelism."><code class="xref py py-mod docutils literal"><span class="pre">threading</span></code></a>
module.</p>
<p>The module implements three types of queue, which differ only in the order in
which the entries are retrieved.  In a <abbr title="first-in, first-out">FIFO</abbr>
queue, the first tasks added are the first retrieved. In a
<abbr title="last-in, first-out">LIFO</abbr> queue, the most recently added entry is
the first retrieved (operating like a stack).  With a priority queue,
the entries are kept sorted (using the <a class="reference internal" href="heapq.html#module-heapq" title="heapq: Heap queue algorithm (a.k.a. priority queue)."><code class="xref py py-mod docutils literal"><span class="pre">heapq</span></code></a> module) and the
lowest valued entry is retrieved first.</p>
<p>Internally, the module uses locks to temporarily block competing threads;
however, it is not designed to handle reentrancy within a thread.</p>
<p>The <a class="reference internal" href="#module-queue" title="queue: A synchronized queue class."><code class="xref py py-mod docutils literal"><span class="pre">queue</span></code></a> module defines the following classes and exceptions:</p>
<dl class="class">
<dt id="queue.Queue">
<em class="property">class </em><code class="descclassname">queue.</code><code class="descname">Queue</code><span class="sig-paren">(</span><em>maxsize=0</em><span class="sig-paren">)</span><a class="headerlink" href="#queue.Queue" title="Permalink to this definition">¶</a></dt>
<dd><p>Constructor for a <abbr title="first-in, first-out">FIFO</abbr> queue.  <em>maxsize</em> is
an integer that sets the upperbound
limit on the number of items that can be placed in the queue.  Insertion will
block once this size has been reached, until queue items are consumed.  If
<em>maxsize</em> is less than or equal to zero, the queue size is infinite.</p>
</dd></dl>

<dl class="class">
<dt id="queue.LifoQueue">
<em class="property">class </em><code class="descclassname">queue.</code><code class="descname">LifoQueue</code><span class="sig-paren">(</span><em>maxsize=0</em><span class="sig-paren">)</span><a class="headerlink" href="#queue.LifoQueue" title="Permalink to this definition">¶</a></dt>
<dd><p>Constructor for a <abbr title="last-in, first-out">LIFO</abbr> queue.  <em>maxsize</em> is
an integer that sets the upperbound
limit on the number of items that can be placed in the queue.  Insertion will
block once this size has been reached, until queue items are consumed.  If
<em>maxsize</em> is less than or equal to zero, the queue size is infinite.</p>
</dd></dl>

<dl class="class">
<dt id="queue.PriorityQueue">
<em class="property">class </em><code class="descclassname">queue.</code><code class="descname">PriorityQueue</code><span class="sig-paren">(</span><em>maxsize=0</em><span class="sig-paren">)</span><a class="headerlink" href="#queue.PriorityQueue" title="Permalink to this definition">¶</a></dt>
<dd><p>Constructor for a priority queue.  <em>maxsize</em> is an integer that sets the upperbound
limit on the number of items that can be placed in the queue.  Insertion will
block once this size has been reached, until queue items are consumed.  If
<em>maxsize</em> is less than or equal to zero, the queue size is infinite.</p>
<p>The lowest valued entries are retrieved first (the lowest valued entry is the
one returned by <code class="docutils literal"><span class="pre">sorted(list(entries))[0]</span></code>).  A typical pattern for entries
is a tuple in the form: <code class="docutils literal"><span class="pre">(priority_number,</span> <span class="pre">data)</span></code>.</p>
</dd></dl>

<dl class="exception">
<dt id="queue.Empty">
<em class="property">exception </em><code class="descclassname">queue.</code><code class="descname">Empty</code><a class="headerlink" href="#queue.Empty" title="Permalink to this definition">¶</a></dt>
<dd><p>Exception raised when non-blocking <a class="reference internal" href="#queue.Queue.get" title="queue.Queue.get"><code class="xref py py-meth docutils literal"><span class="pre">get()</span></code></a> (or
<a class="reference internal" href="#queue.Queue.get_nowait" title="queue.Queue.get_nowait"><code class="xref py py-meth docutils literal"><span class="pre">get_nowait()</span></code></a>) is called
on a <a class="reference internal" href="#queue.Queue" title="queue.Queue"><code class="xref py py-class docutils literal"><span class="pre">Queue</span></code></a> object which is empty.</p>
</dd></dl>

<dl class="exception">
<dt id="queue.Full">
<em class="property">exception </em><code class="descclassname">queue.</code><code class="descname">Full</code><a class="headerlink" href="#queue.Full" title="Permalink to this definition">¶</a></dt>
<dd><p>Exception raised when non-blocking <a class="reference internal" href="#queue.Queue.put" title="queue.Queue.put"><code class="xref py py-meth docutils literal"><span class="pre">put()</span></code></a> (or
<a class="reference internal" href="#queue.Queue.put_nowait" title="queue.Queue.put_nowait"><code class="xref py py-meth docutils literal"><span class="pre">put_nowait()</span></code></a>) is called
on a <a class="reference internal" href="#queue.Queue" title="queue.Queue"><code class="xref py py-class docutils literal"><span class="pre">Queue</span></code></a> object which is full.</p>
</dd></dl>

<div class="section" id="queue-objects">
<span id="queueobjects"></span><h2>17.7.1. Queue Objects<a class="headerlink" href="#queue-objects" title="Permalink to this headline">¶</a></h2>
<p>Queue objects (<a class="reference internal" href="#queue.Queue" title="queue.Queue"><code class="xref py py-class docutils literal"><span class="pre">Queue</span></code></a>, <a class="reference internal" href="#queue.LifoQueue" title="queue.LifoQueue"><code class="xref py py-class docutils literal"><span class="pre">LifoQueue</span></code></a>, or <a class="reference internal" href="#queue.PriorityQueue" title="queue.PriorityQueue"><code class="xref py py-class docutils literal"><span class="pre">PriorityQueue</span></code></a>)
provide the public methods described below.</p>
<dl class="method">
<dt id="queue.Queue.qsize">
<code class="descclassname">Queue.</code><code class="descname">qsize</code><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#queue.Queue.qsize" title="Permalink to this definition">¶</a></dt>
<dd><p>Return the approximate size of the queue.  Note, qsize() &gt; 0 doesn’t
guarantee that a subsequent get() will not block, nor will qsize() &lt; maxsize
guarantee that put() will not block.</p>
</dd></dl>

<dl class="method">
<dt id="queue.Queue.empty">
<code class="descclassname">Queue.</code><code class="descname">empty</code><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#queue.Queue.empty" title="Permalink to this definition">¶</a></dt>
<dd><p>Return <code class="docutils literal"><span class="pre">True</span></code> if the queue is empty, <code class="docutils literal"><span class="pre">False</span></code> otherwise.  If empty()
returns <code class="docutils literal"><span class="pre">True</span></code> it doesn’t guarantee that a subsequent call to put()
will not block.  Similarly, if empty() returns <code class="docutils literal"><span class="pre">False</span></code> it doesn’t
guarantee that a subsequent call to get() will not block.</p>
</dd></dl>

<dl class="method">
<dt id="queue.Queue.full">
<code class="descclassname">Queue.</code><code class="descname">full</code><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#queue.Queue.full" title="Permalink to this definition">¶</a></dt>
<dd><p>Return <code class="docutils literal"><span class="pre">True</span></code> if the queue is full, <code class="docutils literal"><span class="pre">False</span></code> otherwise.  If full()
returns <code class="docutils literal"><span class="pre">True</span></code> it doesn’t guarantee that a subsequent call to get()
will not block.  Similarly, if full() returns <code class="docutils literal"><span class="pre">False</span></code> it doesn’t
guarantee that a subsequent call to put() will not block.</p>
</dd></dl>

<dl class="method">
<dt id="queue.Queue.put">
<code class="descclassname">Queue.</code><code class="descname">put</code><span class="sig-paren">(</span><em>item</em>, <em>block=True</em>, <em>timeout=None</em><span class="sig-paren">)</span><a class="headerlink" href="#queue.Queue.put" title="Permalink to this definition">¶</a></dt>
<dd><p>Put <em>item</em> into the queue. If optional args <em>block</em> is true and <em>timeout</em> is
<code class="docutils literal"><span class="pre">None</span></code> (the default), block if necessary until a free slot is available. If
<em>timeout</em> is a positive number, it blocks at most <em>timeout</em> seconds and raises
the <a class="reference internal" href="#queue.Full" title="queue.Full"><code class="xref py py-exc docutils literal"><span class="pre">Full</span></code></a> exception if no free slot was available within that time.
Otherwise (<em>block</em> is false), put an item on the queue if a free slot is
immediately available, else raise the <a class="reference internal" href="#queue.Full" title="queue.Full"><code class="xref py py-exc docutils literal"><span class="pre">Full</span></code></a> exception (<em>timeout</em> is
ignored in that case).</p>
</dd></dl>

<dl class="method">
<dt id="queue.Queue.put_nowait">
<code class="descclassname">Queue.</code><code class="descname">put_nowait</code><span class="sig-paren">(</span><em>item</em><span class="sig-paren">)</span><a class="headerlink" href="#queue.Queue.put_nowait" title="Permalink to this definition">¶</a></dt>
<dd><p>Equivalent to <code class="docutils literal"><span class="pre">put(item,</span> <span class="pre">False)</span></code>.</p>
</dd></dl>

<dl class="method">
<dt id="queue.Queue.get">
<code class="descclassname">Queue.</code><code class="descname">get</code><span class="sig-paren">(</span><em>block=True</em>, <em>timeout=None</em><span class="sig-paren">)</span><a class="headerlink" href="#queue.Queue.get" title="Permalink to this definition">¶</a></dt>
<dd><p>Remove and return an item from the queue. If optional args <em>block</em> is true and
<em>timeout</em> is <code class="docutils literal"><span class="pre">None</span></code> (the default), block if necessary until an item is available.
If <em>timeout</em> is a positive number, it blocks at most <em>timeout</em> seconds and
raises the <a class="reference internal" href="#queue.Empty" title="queue.Empty"><code class="xref py py-exc docutils literal"><span class="pre">Empty</span></code></a> exception if no item was available within that time.
Otherwise (<em>block</em> is false), return an item if one is immediately available,
else raise the <a class="reference internal" href="#queue.Empty" title="queue.Empty"><code class="xref py py-exc docutils literal"><span class="pre">Empty</span></code></a> exception (<em>timeout</em> is ignored in that case).</p>
</dd></dl>

<dl class="method">
<dt id="queue.Queue.get_nowait">
<code class="descclassname">Queue.</code><code class="descname">get_nowait</code><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#queue.Queue.get_nowait" title="Permalink to this definition">¶</a></dt>
<dd><p>Equivalent to <code class="docutils literal"><span class="pre">get(False)</span></code>.</p>
</dd></dl>

<p>Two methods are offered to support tracking whether enqueued tasks have been
fully processed by daemon consumer threads.</p>
<dl class="method">
<dt id="queue.Queue.task_done">
<code class="descclassname">Queue.</code><code class="descname">task_done</code><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#queue.Queue.task_done" title="Permalink to this definition">¶</a></dt>
<dd><p>Indicate that a formerly enqueued task is complete.  Used by queue consumer
threads.  For each <a class="reference internal" href="#queue.Queue.get" title="queue.Queue.get"><code class="xref py py-meth docutils literal"><span class="pre">get()</span></code></a> used to fetch a task, a subsequent call to
<a class="reference internal" href="#queue.Queue.task_done" title="queue.Queue.task_done"><code class="xref py py-meth docutils literal"><span class="pre">task_done()</span></code></a> tells the queue that the processing on the task is complete.</p>
<p>If a <a class="reference internal" href="#queue.Queue.join" title="queue.Queue.join"><code class="xref py py-meth docutils literal"><span class="pre">join()</span></code></a> is currently blocking, it will resume when all items have been
processed (meaning that a <a class="reference internal" href="#queue.Queue.task_done" title="queue.Queue.task_done"><code class="xref py py-meth docutils literal"><span class="pre">task_done()</span></code></a> call was received for every item
that had been <a class="reference internal" href="#queue.Queue.put" title="queue.Queue.put"><code class="xref py py-meth docutils literal"><span class="pre">put()</span></code></a> into the queue).</p>
<p>Raises a <a class="reference internal" href="exceptions.html#ValueError" title="ValueError"><code class="xref py py-exc docutils literal"><span class="pre">ValueError</span></code></a> if called more times than there were items placed in
the queue.</p>
</dd></dl>

<dl class="method">
<dt id="queue.Queue.join">
<code class="descclassname">Queue.</code><code class="descname">join</code><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#queue.Queue.join" title="Permalink to this definition">¶</a></dt>
<dd><p>Blocks until all items in the queue have been gotten and processed.</p>
<p>The count of unfinished tasks goes up whenever an item is added to the queue.
The count goes down whenever a consumer thread calls <a class="reference internal" href="#queue.Queue.task_done" title="queue.Queue.task_done"><code class="xref py py-meth docutils literal"><span class="pre">task_done()</span></code></a> to
indicate that the item was retrieved and all work on it is complete. When the
count of unfinished tasks drops to zero, <a class="reference internal" href="#queue.Queue.join" title="queue.Queue.join"><code class="xref py py-meth docutils literal"><span class="pre">join()</span></code></a> unblocks.</p>
</dd></dl>

<p>Example of how to wait for enqueued tasks to be completed:</p>
<div class="highlight-python3"><div class="highlight"><pre><span></span><span class="k">def</span> <span class="nf">worker</span><span class="p">():</span>
    <span class="k">while</span> <span class="kc">True</span><span class="p">:</span>
        <span class="n">item</span> <span class="o">=</span> <span class="n">q</span><span class="o">.</span><span class="n">get</span><span class="p">()</span>
        <span class="k">if</span> <span class="n">item</span> <span class="ow">is</span> <span class="kc">None</span><span class="p">:</span>
            <span class="k">break</span>
        <span class="n">do_work</span><span class="p">(</span><span class="n">item</span><span class="p">)</span>
        <span class="n">q</span><span class="o">.</span><span class="n">task_done</span><span class="p">()</span>

<span class="n">q</span> <span class="o">=</span> <span class="n">queue</span><span class="o">.</span><span class="n">Queue</span><span class="p">()</span>
<span class="n">threads</span> <span class="o">=</span> <span class="p">[]</span>
<span class="k">for</span> <span class="n">i</span> <span class="ow">in</span> <span class="nb">range</span><span class="p">(</span><span class="n">num_worker_threads</span><span class="p">):</span>
    <span class="n">t</span> <span class="o">=</span> <span class="n">threading</span><span class="o">.</span><span class="n">Thread</span><span class="p">(</span><span class="n">target</span><span class="o">=</span><span class="n">worker</span><span class="p">)</span>
    <span class="n">t</span><span class="o">.</span><span class="n">start</span><span class="p">()</span>
    <span class="n">threads</span><span class="o">.</span><span class="n">append</span><span class="p">(</span><span class="n">t</span><span class="p">)</span>

<span class="k">for</span> <span class="n">item</span> <span class="ow">in</span> <span class="n">source</span><span class="p">():</span>
    <span class="n">q</span><span class="o">.</span><span class="n">put</span><span class="p">(</span><span class="n">item</span><span class="p">)</span>

<span class="c1"># block until all tasks are done</span>
<span class="n">q</span><span class="o">.</span><span class="n">join</span><span class="p">()</span>

<span class="c1"># stop workers</span>
<span class="k">for</span> <span class="n">i</span> <span class="ow">in</span> <span class="nb">range</span><span class="p">(</span><span class="n">num_worker_threads</span><span class="p">):</span>
    <span class="n">q</span><span class="o">.</span><span class="n">put</span><span class="p">(</span><span class="kc">None</span><span class="p">)</span>
<span class="k">for</span> <span class="n">t</span> <span class="ow">in</span> <span class="n">threads</span><span class="p">:</span>
    <span class="n">t</span><span class="o">.</span><span class="n">join</span><span class="p">()</span>
</pre></div>
</div>
<div class="admonition seealso">
<p class="first admonition-title">See also</p>
<dl class="docutils">
<dt>Class <a class="reference internal" href="multiprocessing.html#multiprocessing.Queue" title="multiprocessing.Queue"><code class="xref py py-class docutils literal"><span class="pre">multiprocessing.Queue</span></code></a></dt>
<dd>A queue class for use in a multi-processing (rather than multi-threading)
context.</dd>
</dl>
<p class="last"><a class="reference internal" href="collections.html#collections.deque" title="collections.deque"><code class="xref py py-class docutils literal"><span class="pre">collections.deque</span></code></a> is an alternative implementation of unbounded
queues with fast atomic <a class="reference internal" href="collections.html#collections.deque.append" title="collections.deque.append"><code class="xref py py-meth docutils literal"><span class="pre">append()</span></code></a> and
<a class="reference internal" href="collections.html#collections.deque.popleft" title="collections.deque.popleft"><code class="xref py py-meth docutils literal"><span class="pre">popleft()</span></code></a> operations that do not require locking.</p>
</div>
</div>
</div>


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  <h3><a href="../contents.html">Table Of Contents</a></h3>
  <ul>
<li><a class="reference internal" href="#">17.7. <code class="docutils literal"><span class="pre">queue</span></code> — A synchronized queue class</a><ul>
<li><a class="reference internal" href="#queue-objects">17.7.1. Queue Objects</a></li>
</ul>
</li>
</ul>

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